Atmospheric formaldehyde production on early Mars leading to a potential formation of bio-important molecules

Formaldehyde (H2CO) is a critical precursor for the abiotic formation of biomolecules, including amino acids and sugars, which are the building blocks of proteins and RNA. Geomorphological and geochemical evidence on Mars indicates a temperate environment compatible with the existence of surface liquid water during its early history at 3.8–3.6 billion years ago (Ga), which was maintained by the warming effect of reducing gases, such as H2. However, it remains uncertain whether such a temperate and weakly reducing surface environment on early Mars was suitable for producing H2CO. In this study, we investigated the atmospheric production of H2CO on early Mars using a 1-D photochemical model assuming a thick CO2-dominated atmosphere with H2 and CO. Our results show that a continuous supply of atmospheric H2CO can be used to form various organic compounds, including amino acids and sugars. This could be a possible origin for the organic matter observed on the Martian surface. Given the previously reported conversion rate from H2CO into ribose, the calculated H2CO deposition flux suggests a continuous supply of bio-important sugars on early Mars, particularly during the Noachian and early Hesperian periods.


Atmospheric formaldehyde production on early Mars leading to a potential formation of bio-important molecules
Shungo Koyama 1* , Arihiro Kamada 1 , Yoshihiro Furukawa 1 , Naoki Terada 1 , Yuki Nakamura 2 , Tatsuya Yoshida 1 , Takeshi Kuroda 1,3 & Ann Carine Vandaele 4 Formaldehyde (H 2 CO) is a critical precursor for the abiotic formation of biomolecules, including amino acids and sugars, which are the building blocks of proteins and RNA.Geomorphological and geochemical evidence on Mars indicates a temperate environment compatible with the existence of surface liquid water during its early history at 3.8-3.6 billion years ago (Ga), which was maintained by the warming effect of reducing gases, such as H 2 .However, it remains uncertain whether such a temperate and weakly reducing surface environment on early Mars was suitable for producing H 2 CO.In this study, we investigated the atmospheric production of H 2 CO on early Mars using a 1-D photochemical model assuming a thick CO 2 -dominated atmosphere with H 2 and CO.Our results show that a continuous supply of atmospheric H 2 CO can be used to form various organic compounds, including amino acids and sugars.This could be a possible origin for the organic matter observed on the Martian surface.Given the previously reported conversion rate from H 2 CO into ribose, the calculated H 2 CO deposition flux suggests a continuous supply of bio-important sugars on early Mars, particularly during the Noachian and early Hesperian periods.
Present-day Mars is extremely cold and dry, but many geomorphological and geochemical evidence, such as valley networks, suggest an active water cycle at 3.8-3.6Ga 1 .The detection of phyllosilicates over the Noachian terrain also supports the existence of widespread liquid water on early Mars 2 .The habitability of Mars has been of great interest that triggered previous and ongoing Martian explorations.Although water is a probable requirement of Martian habitability, this molecule is not an organic compound in genomic and catalytic bio-molecules that supports the fundamentals of life.Investigations on the organic synthesis on ancient Mars fill the gap by verifying the possibility, environment, and age of the chemical evolution to potential ancient Martian life.
Formaldehyde (H 2 CO) is simple organic matter that can be formed through various chemical reactions in planetary atmospheres.H 2 CO is a highly soluble and reactive molecule, and thus has the potential to play a significant role in the abiotic formation of bioorganic molecules 3 .For example, amino acids are formed by reactions involving H 2 CO, NH 3 , and HCN via the Strecker reaction 4 .Additionally, in ammonia-involving formose-type reactions, the condensation of H 2 CO with NH 3 yields various amino acids 5,6 .The formose reaction is a thermally driven aqueous process that generates many sugars from H 2 CO, including ribose, a fundamental building block of RNA that is regarded as a key molecule for the origin of life [7][8][9] .Therefore, determining whether the surface environments on early terrestrial planets fostered the production of H 2 CO is crucial for understanding prebiotic chemical evolution to the origin of life.
The evidence that supports the existence of liquid water have led many scientists to imagine a warm early Martian climate as it is on Earth 1,2 .Previous numerical studies have attempted to reproduce warm early Mars; however, 3-D global circulation model (GCM) studies have not been able to reproduce the continuous presence of liquid water on the surface with CO 2 -H 2 O atmospheres 10,11 .To reconcile this geomorphological evidence, episodic melting scenarios driven by the supply of reducing gases through volcanic outgassing or meteorite impacts have been proposed [12][13][14][15] .
Pinto et al. 16 provided an estimate for the photochemical production of H 2 CO in the N 2 -dominated atmosphere of primitive Earth, with the number density of H 2 CO near the surface approximated to be ~ 10 8 cm −3 .A similar amount of H 2 CO production was also predicted for early Earth conditions by Harman et al. 17 .However, the production of H 2 CO in a CO 2 -dominated atmosphere on early Mars has not yet been thoroughly investigated.Understanding its production on early Mars can provide insights into the potential of life on the planet.
In this study, we investigated the production of H 2 CO in a thick CO 2 -dominated atmosphere containing H 2 and CO on early Mars using a 1-D photochemical model.We then estimated ribose production using our simulation results and experimental data.

Model description
To calculate the atmospheric production of H 2 CO, we adapt a one-dimensional photochemical model, PRO-TEUS (Photochemical and RaiatiOn Transport model for Extensive USe), detailed by Nakamura et al. 18 , for early Martian conditions.This model has been successfully applied to other planetary atmospheres, such as the Jovian ionosphere 19 and present-day Martian atmosphere 20 .It solves the continuity equations involving chemical reactions and vertical transport until the profiles of each species reach a steady state.We consider 63 chemical reactions (Supplementary + in a 2-bar CO 2 -dominated atmosphere.Though the atmospheric surface pressure of early Mars is still not well constrained, 3-D global circulation model studies suggest that a 2 bar CO 2 atmosphere with a few percentages of reducing gas is required for a warm climate 14,15 .The exobase altitude is defined as a pressure level of 10 −9 mbar.We utilize the H 2 O vapor number density and temperature profiles up to ~ 60 km (Fig. 1) from the 2-bar global mean results with an obliquity of 40° computed using a 3-D paleo-Mars global climate model 15,21 .The global climate model assumed three scenarios of an atmosphere containing 0, 3, and 6% H 2 .For the H 2 O density profiles above ~ 60 km, we assume the same mixing ratio up to the exobase, assuming the effect of cold trap.The sensitivity to the H 2 O vapor content in the atmosphere is discussed in Results section.For temperature, we assume isothermal up to the lower boundary of the thermosphere (6 × 10 −3 Pa) considering radiative equilibrium and then extrapolate it into the thermosphere 22,23 .The exobase temperature is fixed at 800 K, corresponding to 10 × EUV at 3.8 Ga 24 .We adopt the solar spectrum from 3.8 Ga estimated by Claire et al. 25 .We use the updated H 2 O absorption cross section measured by Ranjan et al 26 .We assume the up-to-date absorption cross sections for all the species to the best of our knowledge.References for cross sections of all species at each wavelength are presented in Nakamura et al. 18 .The CO 2 + concentration profile calculated by the ionosphere photochemistry model 27  www.nature.com/scientificreports/us to calculate the escape flux of hydrogen 23 .Although the density of CO 2 + in the early Martian atmosphere has uncertainty, its profile has little impact on the result, because the escape flux of hydrogen is limited by H 2 diffusion from the lower atmosphere.We adopt eddy diffusion coefficient profiles using the typical formula adapted for other planets than Earth 28 .The vertical temperature, H 2 O number density, and eddy diffusion coefficient profiles are shown in Fig. 1.The temperature and water vapor profiles of the H 2 0% scenario shown in Fig. 1 are used to calculate the H 2 CO production under the conditions of 0.1, 0.01, 0.001, and 0.0001% H 2 .
For the upper boundary, we assume Jeans escape of H and H 2 and fix the O escape rate at 2.6 × 10 8 cm −2 s −1 corresponding to 10 × EUV conditions 24 .The O escape rate does not have a large impact on the results because it does not control O 2 abundance in the lower atmosphere where H 2 CO is dominantly produced.Deposition velocities are applied to H 2 O 2 , HO 2 , H 2 CO, HCO, OH, O, and H 29,30 .The deposition velocities of each species are shown in Supplementary Table S2 online.We compute the dry deposition of H 2 CO by imposing the deposition velocity.This model also includes the rainout of H 2 CO throughout the atmosphere using the same parameterization as Hu et al 28 : where k R is rainout frequency, f R is a reduction factor which is an adjustable parameter to represent the reduction relative to Earth's hydrological cycle, n H 2 O is the number density of H 2 O, k H 2 O is the precipitation rate assumed to be 2 × 10 -6 s −1 , N A is Avogadro's constant, L is the liquid water content assumed to be 1 g m −3 , H′ is the effective Henry's Law constant assumed to be 1.3 × 10 4 M atm −1 taken from Giorgi & Chameides 31 , R is gas constant, and T is temperature.The rainout rate is then obtained by multiplying k R by number density of H 2 CO.We assume f R to be 1, assuming that early Mars had a hydrological cycle similar to Earth's.Sensitivity to f R is discussed in Results section.The boundary conditions for H 2 and CO are fixed in the calculation of H 2 CO production in the results.We impose H 2 outgassing and CO deposition velocity as a boundary condition to determine the possible range of H 2 and CO mixing ratio in the following section.

Background H 2 and CO atmospheric conditions
Potential sources of H 2 gas on early Mars include volcanic degassing 29 , meteorite impacts [32][33][34] , and serpentinization 35 .The upper limit of the H 2 outgassing rate is estimated to be 8 × 10 11 cm −2 s −1 considering the supplies from volcanism and serpentinization 29 .We computed H 2 mixing ratios in a background 2-bar CO 2 atmosphere for various H 2 outgassing rates to investigate the plausibility of CO 2 atmospheres enriched in H 2 , assuming a fixed CO deposition velocity of 10 −8 cm s −129 (see Supplementary Fig. S1 online).An H 2 outgassing rate of ~ 5 × 10 11 cm −2 s −1 yields a 5-6% H 2 mixing ratio in a 2-bar CO 2 atmosphere.This result is consistent with that of Batalha et al. 29 , in which an H 2 outgassing rate of 8 × 10 11 cm −2 s −1 yields a ~ 5% H 2 mixing ratio in a 3-bar CO 2 atmosphere.The minimum value of the H 2 mixing ratio is ~ 1 × 10 -6 when assuming the absence of H 2 degassing.Furthermore, Chassefiere et al. 35 suggested that serpentinization-derived CH 4 trapped in the cryosphere could have been released into the atmosphere, producing a transient 1-2 bar CO 2 atmosphere containing 10-20% H 2 gas.Thus, a 6% H 2 mixing ratio in a background 2-bar CO 2 atmosphere is plausible.This study considers a range of H 2 mixing ratios from 1 × 10 −6 to 0.06.
The abundance of CO on early Mars is not well constrained.A dense and cold CO 2 atmosphere is likely to enter the CO runaway state 23 .The CO and O liberated from CO 2 photolysis no longer recombine because of the lack of odd hydrogen species that catalyze CO 2 recombination.Moreover, because the equilibrium timescale of CO is relatively long, ranging from several million to several hundred million years 23 , it is insufficient to consider only a single steady state condition.Therefore, it is necessary to vary the CO mixing ratio as a parameter within a specific range.
To determine the possible CO range, we calculated the CO mixing ratios over a wide range of CO deposition velocities for 0, 3, and 6% H 2 cases.Different temperatures and H 2 O profiles are used for each H 2 case obtained from the GCM results 15 .We assume a free lower boundary condition for H 2 in the 0% H 2 case, whereas the number density is fixed for the 3 and 6% H 2 conditions.
As a result, the 0% H 2 case enters a CO runaway state with a low CO deposition velocity of < 10 −10 cm s −1 (see Supplementary Fig. S2 online) as suggested by the previous photochemical model study of early Mars conditions 36 .Conversely, warmer atmospheres containing 3 or 6% H 2 have a maximum CO mixing ratio of ~ 1% because sufficient H 2 O vapor promotes CO 2 recombination.The CO deposition velocity on an abiotic ocean planet is estimated to be 10 −9 -10 −8 cm s −137,38 ; however, there is no known efficient process to remove CO at the surface without the ocean 28 .Considering that ancient Mars experienced episodic cold and warm climates 1 , a CO 2 atmosphere would have been in a CO runaway state during the ice age 36 , while the atmosphere would have been stable with a minimum CO mixing ratio of 1% in warmer climates.Based on these results, the possible range of CO on early Mars should be from 1 to 50%.

Photochemical production of formaldehyde
The present 1-D photochemical model shows that H 2 CO forms at number densities of ~ 5 × 10 9 and ~ 7 × 10 −2 cm −3 near the surface under 6% H 2 condition and 0.01% H 2 condition, respectively (Fig. 2).Production in a 6% H 2 atmosphere is substantial; approximately 50 times higher than that estimated for Earth's Hadean atmosphere 16 .
H 2 CO is predominantly formed through a radical-radical reaction of two HCO molecules 39 : These reactions imply that the production of H 2 CO decreases with O 2 because O 2 reacts with HCO through R3, thereby reducing the rate of R1.
The H 2 CO production in the 6% H 2 atmosphere is notably higher than that under lower H 2 conditions.This results from the discrepancy in the O 2 abundance in the lower atmosphere between these two conditions.There is a sharp decline in the O 2 density below 60 km for 6% H 2 (Fig. 2).The lack of O 2 in the 6% H 2 atmosphere decreased the reaction rate of HCO with O 2 (R3).Consequently, a larger amount of HCO remains near the surface, ultimately increasing the amount of H 2 CO via reaction R1.Two mechanisms below contribute to the decrease in O 2 .
The first mechanism is driven by H atoms at altitudes greater than 100 km.The source of O 2 near the surface is the downward transport of O 2 liberated from CO 2 photolysis at high altitudes.The significant difference between the 6% and 0.01% H 2 cases is the number of H atoms at altitudes above 100 km.H 2 is transported upward and photolyzed into H atoms by the solar UV flux, producing more H in the 6% H 2 case.O 2 reacts with H at high altitudes and is converted back into CO 2 via the following reaction: The altitude profiles of the reaction rate of R4 between the 6% and 0.01% H 2 cases indicate that R4 shifted upward under the 6% H 2 condition (Fig. 3).The loss of O 2 by H atoms through R4 above ~ 150 km results in a decrease in O 2 downward flux below ~ 135 km, as shown in Fig. 4.
The second mechanism is driven by the HCO catalytic cycle.In the present-day Martian atmosphere, odd hydrogen species act as catalysts to recombine CO and O into CO 2 through the following cycle 40 : The net result is the recombination of CO 2 .In a dense CO 2 atmosphere, HCO catalytic reactions are responsible for converting H to HO 2 in addition to R4: www.nature.com/scientificreports/ This is because a larger amount of CO is formed in a denser atmosphere owing to the lack of amounts of odd hydrogen, while O 2 is more abundant than CO in the present-day Martian atmosphere.The reaction rate profile of R3 in a 6% H 2 atmosphere indicates that the HCO catalytic cycle dominates at ~ 50-60 km (Fig. 3), where the O 2 density declines sharply (Fig. 2).This cycle significantly reduces O 2 in the lower atmosphere below 50-60 km.

Deposition of formaldehyde
A 6% H 2 mixing ratio enables the presence of an ocean in a warm climate 15 .In a warm environment with an ocean and a CO deposition velocity of 10 −8 -10 −9 cm s −137,38 , the CO mixing ratio is ~ 1%, as shown in the 3% and 6% H 2 cases in Supplementary Fig. S2.Consequently, the deposition flux of H 2 CO into the surface liquid water is approximately 3 × 10 9 cm −2 s −1 in a warm climate.
When the mixing ratio of H 2 and CO decreases, the deposition of H 2 CO decreases.However, the H 2 CO decrease is not gradual, and there is a respective large drop with the decrease in H 2 and CO.Large amounts of H 2 CO are deposited in an atmosphere containing either > 0.1% H 2 or > 50% CO (Fig. 5).
The high-altitude H generates a large drop between 0.01 and 0.1% H 2 , as shown in Fig. 5.The large decrease between 10 and 50% CO is associated with the HCO catalytic cycle (Fig. 5).Increasing the CO produces more HCO (R2), thereby removing O 2 in the lower atmosphere (R3).In an atmosphere containing 10% CO, a steady state is reached with more O 2 and less HCO.Conversely, the atmosphere containing 50% CO reaches a steady state with less O 2 and more HCO.Therefore, in an atmosphere with H 2 < 0.01%, more H 2 CO is produced with 50% CO.

Formaldehyde deposition in more abundant H 2 and H 2 O conditions
We first assessed the impact of a higher H 2 mixing ratio than 6% on H 2 CO production, with the CO mixing ratio fixed at 1% and H 2 O and temperature profiles from the 6% H 2 GCM results 15 .This assumption may not represent early Martian conditions, but it allows the estimation of the upper limit of H 2 CO production in a CO 2 -dominated atmosphere on early Mars and provides insights into an exoplanet analog for a CO 2 -dominated atmosphere enriched with H 2 .As shown in Fig. 6, increasing the H 2 mixing ratio from 1 to 20% increased the H 2 CO deposition slightly.However, the deposition flux does not exceed 5 × 10 9 cm −2 s −1 .This result implies that H 2 CO production under early Martian atmospheric conditions would be close to its maximum in H 2 -rich CO 2 -dominated atmospheres.Subsequently, we multiplied the H 2 O profile of the 6% H 2 case by a factor ranging from 0.01 to 100 while maintaining the H 2 and CO mixing ratios at 6% and 1%, respectively, with the temperature profile of the 6% H 2 case.This approach accounts for spatiotemporal variations in water vapor content on a global scale.H 2 O vapor changes by ~ 10 times over the global scale in the global climate model results 21 .Considering the seasonal difference, changes by ~ 2 orders of magnitude are reasonable for early Mars conditions.As an upper and lower limit, we changed the amount of H 2 O vapor by 4 orders of magnitude.This parameter survey may also be useful for exoplanets' environment.A higher water vapor content significantly increases H 2 CO production (Fig. 6b).The H 2 CO deposition flux increases up to ~ 1 × 10 12 cm −2 s −1 with a 100 times higher H 2 O density.Under conditions of abundant H 2 O, additional H atoms are generated through H 2 O photolysis, resulting in increased HCO formation through R2 and, consequently, greater H 2 CO production through R1.In addition, an increase in precipitation also contributes to the increased rainout rate of H 2 CO.This indicates that in a warm climate with 6% H 2 , the limiting factor for H 2 CO production is not H 2 but the abundance of H 2 O.The timescale for the change in H 2 CO formation due to variations in water vapor content is a few years in our calculation.This result suggests that H 2 CO production on early Mars exhibited local variations depending on the availability of water vapor.However, further studies are needed to clarify this effect, as horizontal transport may mitigate differences in H 2 CO abundance.
We also investigated the effect of the reduction factor f R in Eq. ( 1) on H 2 CO deposition flux.The calculated H 2 CO deposition fluxes with a reduction factor of 0.1, 0.5, and 1 are shown in Supplementary Fig. S4.In this calculation, H 2 and CO mixing ratios are fixed at 6% and 1%, respectively, with temperature and H 2 O profiles of the 6% H 2 case.When f R is set to 0.1, it is reduced to 8 × 10 8 cm −2 s −1 , approximately 1/4 of the value when f R is 1.Although it is not yet constrained, GCM results suggest that globally averaged precipitation on early Mars may have been 10 times smaller than on Earth 15 .Further modeling studies combining photochemistry with GCM would be helpful for a more accurate estimation of the rainout rate.

Formation of formaldehyde throughout Mars' history
The deposition rate of H 2 CO reaches the order of 10 8 or 10 9 cm −2 s −1 under conditions where the mixing ratio of H 2 is higher than 0.1%, regardless of the CO mixing ratio (Fig. 5).The H 2 mixing ratio of 0.1% is equivalent to ~ 10 10 cm −2 s −1 of H 2 outgassing rate in a steady state (Supplementary Fig. S1).This rate is comparable to the estimated H 2 degassing rates on present-day Earth, which has an upper mantle of quartz-fayalite-magnetite (QFM) oxidation buffer 29,41 .The H 2 degassing rate on early Mars is unclear.However, Martian meteorites suggest that the mantle was more reduced than the Earth's upper mantle, with oxygen fugacity around the iron-wüstite (IW) buffer 42,43 .Given that the oxygen fugacity of the Martian upper mantle was buffered near IW + 1, the H 2 degassing rate is estimated to be ~ 10 11 cm −2 s −129, 41 .This suggests that the large atmospheric production of H 2 CO continued during past periods of active volcanic degassing, regardless of the CO mixing ratio (Fig. 7).An increase in the H 2 mixing ratio from 0.1 to 6% increases the H 2 CO deposition rate by approximately 10 times the global mean value (Fig. 5).This increase was mainly due to the increase in the number density of H 2 O by 10 times due to the warming effect of increasing H 2 (Fig. 1d).The number density of H 2 O in the atmosphere also differs (e.g., 10 times) depending on the local availability of H 2 O 15 .This difference could provide local H 2 CO deposition rates on Mars that are several tens of times higher and lower than the global mean value (Fig. 6b).
Volcanic degassing would have decreased from the late Hesperian to the Amazonian, decreasing the mixing ratio of H 2 and H 2 O in the atmosphere 42 .A decrease in the H 2 mixing ratio to below 0.1% dramatically decreased the H 2 CO deposition rate by a factor of 10 −10 (Fig. 5).Thus, the late Hesperian to early Amazonian was a transitional period from a high to a meager H 2 CO deposition rate (Fig. 7).

Formation of organic compounds in the ocean
The continuous conversion of CO 2 and CO into highly soluble H 2 CO in the early Martian atmosphere may have transferred carbon from the atmosphere to the ocean.Another mechanism that converts atmospheric carbon into H 2 CO involves iron-rich asteroids/meteorites 44 .Such impacts might have formed H 2 CO, both locally and temporally.The overall impact-induced production would have been smaller than the continuous global production of H 2 CO in the atmosphere through the photochemical reactions presented in this study (see Supplementary text online).Another source of H 2 CO discussed previously is the oxidation of CH 4 with iron oxide, which was proposed to explain the tentative detection of H 2 CO on present-day Mars 45 .
H 2 CO is highly reactive.Carbon transferred from the atmosphere as H 2 CO could further be converted into various organic compounds.One of the most well-known reactions is the formose reaction, in which formaldehyde oligomerizes to form various sugar molecules in alkaline solutions 7 .A recent study found that this type of reaction can form sugars, including ribose, even in neutral solutions 46 .To estimate ribose production in the   .0Ga (bottom panel).In the Noachian and early Hesperian periods (3.8-3.6 Ga), the synthesized H 2 CO in the atmosphere was deposited into the ocean, forming bio-important molecules, such as ribose.In the middle and late Hesperian (3.6-3.0Ga), H 2 CO was sporadically formed.Even in the period when H 2 CO was abundantly formed, subsequent formose reaction does not proceed due to the acidic condition of the water.From the Amazonian to the present (after 3.0 Ga), the production of H 2 CO is deficient, as in the case with H 2 < 0.1% in the middle and late Hesperian.www.nature.com/scientificreports/early Martian ocean, we make the following assumptions: one-third of the surface area was covered by an ocean as suggested by the analysis of the distribution of delta and valleys 47 , the seawater pH at ~ 3.8 Ga in early Mars was near-neutral, as the late Noachian marked a transition period from alkaline to acidic water pH 48 , early Mars had a hydrological cycle similar to Earth's (f R = 1), the atmosphere contained the same fraction of glycolaldehyde as an estimated atmosphere of the early Earth 17 , and the conversion rate of H 2 CO into ribose was ~ 3.5 × 10 −6 mol Rib mol FA −1 estimated by the formose-reaction experiment 46 .By combining the calculated H 2 CO deposition flux of 3 × 10 9 cm −2 s −1 (= 1 × 10 21 m −2 yr −1 ), the annual ribose production in the ocean P rib is estimated to be 4 × 10 4 kg yr −1 as follows: where f H 2 CO is the deposition flux of H 2 CO (m −2 yr −1 ), S sea is the area covered by the ocean (m 2 ), Y rib is the con- version rate of H 2 CO into ribose (mol mol −1 ), M rib is the molar mass of ribose (kg mol −1 ), and N A is Avogadro's constant.When f R is 0.1, the annual ribose production is estimated to be 1 × 10 4 kg yr −1 .These suggest that bioimportant sugars including ribose might have been continuously formed in water bodies on the surface.In this assumption, we disregarded several H 2 CO consumption processes in seawater, including photolysis, hydrolysis, and reactions with other reactive molecules such as ammonia 3,49 .It is unclear whether ammonia was present on early Mars, but it may have transiently been present in the atmosphere or in water on early Mars, potentially due to processes, such as episodic volcanic degassing or impact degassing.Previous studies on early Earth indicated that reducing gases, including ammonia, could be generated through impact events 50,51 .This process may have analogously occurred on early Mars.In the presence of ammonia, the formose reaction forms various nitrogen containing organic matter including proteinogenic amino acids 5 .Nitrogen containing organic matter has been found in a Martian meteorite 52 .The formose reaction also forms refractory organic matter, similar to cometary and meteoritic insoluble organic matter 6 .The photochemical H 2 CO calculated in this study and its following formose reaction may be related to the origin of refractory and non-refractory organic matter found in the 3.5-billion-year-old lacustrine mudstones of Mars 53 and Noachian carbonates in a Martian meteorite 52 .However, it is still difficult to distinguish whether this organic matter was derived from the photochemical H 2 CO.One way to distinguish them is to compare their carbon isotope compositions.The carbon isotopic analysis onboard the Curiosity rover detected an anomalously depleted 13 C in organic matter 54 .The deposition of photochemical H 2 CO which experienced CO 2 photolysis-driven carbon isotope fractionation might explain this depletion 55 .Our future work is to include carbon isotope fractionation in the model and compare it with the isotope observation data.Significant H 2 CO synthesis on the warm Noachian Mars with liquid surface water allowed for the formation of sugars and amino acids (Fig. 7).The formation of H 2 CO would have sporadically continued on Hesperian to early Amazonian Mars, but the transition to an ice-covered and acidic surface environment on Hesperian Mars dramatically decreased the possibility of the formation of the building blocks of life, because the production of sugars and amino acids through the formose-type reaction substantially decreased in acidic water 46,48,56,57 .Therefore, the time period suitable for the formation of bio-important molecules on early Mars might be limited to the Noachian and potentially early Hesperian Mars, the warm climate era before the pH of the surface liquid water became acidic.
Secondary concentration processes are essential for synthesizing bio-important molecules on planetary surfaces.Early Mars may have experienced episodic warm and cold climate periods 33 .During the transition from warm to cold periods, a large amount of oceanic water would have been stored as snow on land; thus, oceanic water would have become concentrated.Such evaporative environments might further promote chemical evolution to form biopolymers, such as proteins and RNAs because the primary reaction that forms these molecules is dehydration reactions 58,59 .These reactions may have been promoted by carbonate and borate, which have been shown to be present on Mars [60][61][62][63][64] .Future studies considering topography and secondary concentration processes are important to elucidate the possibility of RNA synthesis on early Mars.

Conclusions
Formaldehyde (H 2 CO) is a crucial organic matter in the formation of bioorganic molecules such as amino acids and ribose.We investigated the atmospheric production of H 2 CO on early Mars using a one-dimensional photochemical model.We assume a 2-bar background CO 2 -dominated atmosphere with various concentrations of H 2 and CO while adopting temperature and H 2 O profiles from a 3-D paleo-Mars global climate model.Our results show that a larger amount of H 2 leads to a more significant production of H 2 CO owing to the reduction in O 2 abundance in the lower atmosphere.Two mechanisms cause this O 2 reduction: (1) the chemical reaction with H atoms at high altitudes above 100 km, resulting in a decrease in O 2 downward flux, and (2) the HCO catalytic cycle at ~ 50-60 km reducing O 2 in the lower atmosphere below 60 km.In a warm climate, the number density of H 2 CO is ~ 5 × 10 9 cm −3 near the surface, and its deposition into the ocean is 3 × 10 9 cm −2 s −1 assuming that early Mars had a hydrological cycle similar to Earth's.The sensitivity analysis of water vapor implies that H 2 CO production could have varied locally in correlation with the abundance of water vapor.Our results suggest that a continuous supply of H 2 CO could be used to form various organic compounds, including life's building blocks, such as amino acids and sugars.This photochemically produced H 2 CO could be a possible origin for the organic matter observed on the Martian surface.Given the previously reported conversion rate from H 2 CO to ribose, the calculated H 2 CO deposition flux suggests a continuous supply of bio-important sugars on Noachian and early Hesperian Mars. (

Figure 2 .
Figure2.Number density profiles of main species under the 6% H 2 (solid line) and 0.01% H 2 (dashed line) conditions.CO is fixed at 1% in both cases.

Figure 3 .
Figure 3.A comparison of O 2 loss reaction rates per one O 2 molecule (= reaction rate / O 2 number density) under the 6% H 2 (solid line) and 0.01% H 2 (dashed line) conditions.The black and blue lines show the reaction rates of H + O 2 + M → HO 2 + M (R4) and HCO + O 2 → HO 2 + CO (R3), respectively.

Figure 4 .
Figure 4.A comparison of O 2 downward flux profiles under the 6% H 2 (solid line) and 0.01% H 2 (dashed line) conditions.

Figure 5 .
Figure 5. H 2 CO deposition flux as a function of CO and H 2 mixing ratios assuming a 2-bar CO 2 background atmosphere.

Figure 6 .
Figure 6.H 2 CO deposition fluxes as a function of the H 2 mixing ratio (a) and H 2 O factor (b).The temperature profile of the 6% H 2 case is used for both calculations.The bottom panel (b) shows H 2 CO deposition fluxes obtained by multiplying the H 2 O profile of the 6% H 2 case by a factor ranging from 0.01 to 100.

Figure 7 .
Figure 7. Scenario for the H 2 CO production at ca. 3.8-3.6Ga (top panel), ca.3.5-3.0Ga (middle panel), and after ca.3.0Ga (bottom panel).In the Noachian and early Hesperian periods (3.8-3.6 Ga), the synthesized H 2 CO in the atmosphere was deposited into the ocean, forming bio-important molecules, such as ribose.In the middle and late Hesperian (3.6-3.0Ga), H 2 CO was sporadically formed.Even in the period when H 2 CO was abundantly formed, subsequent formose reaction does not proceed due to the acidic condition of the water.From the Amazonian to the present (after 3.0 Ga), the production of H 2 CO is deficient, as in the case with H 2 < 0.1% in the middle and late Hesperian.

3
Figure 7. Scenario for the H 2 CO production at ca. 3.8-3.6Ga (top panel), ca.3.5-3.0Ga (middle panel), and after ca.3.0Ga (bottom panel).In the Noachian and early Hesperian periods (3.8-3.6 Ga), the synthesized H 2 CO in the atmosphere was deposited into the ocean, forming bio-important molecules, such as ribose.In the middle and late Hesperian (3.6-3.0Ga), H 2 CO was sporadically formed.Even in the period when H 2 CO was abundantly formed, subsequent formose reaction does not proceed due to the acidic condition of the water.From the Amazonian to the present (after 3.0 Ga), the production of H 2 CO is deficient, as in the case with H 2 < 0.1% in the middle and late Hesperian. https://doi.org/10.1038/s41598-024-52718-9 Table S1 online) for 8 neutrals: CO 2 , CO, H 2 , H 2 O, O 2 , H 2 O 2 , O 3 , H 2 CO, 6 radicals: H, O, OH, HO 2 , O( 1 D), HCO, and an ion of CO 2 is fixed at the same pressure altitude as present-day Mars to represent the dissociation reaction of H 2 with CO 2 + to produce atomic H in the upper atmosphere, allowing The dominant production path of HCO is a three-body reaction of H and CO:where M represents the background gas, mainly CO 2 in this model.In the early Martian atmosphere, H and CO in R2 are derived from H 2 O and CO 2 photolysis, respectively.HCO is dominantly destroyed by a reaction with O 2 : R1) HCO + HCO → H 2 CO + CO.Vol:.(1234567890)Scientific Reports | (2024) 14:2397 | https://doi.org/10.1038/s41598-024-52718-9www.nature.com/scientificreports/